Catholics believe that sexuality has a deep, fundamental purpose, designed by God.
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[M]ost Catholics and almost all Protestants do not even dimly understand the distinctive Catholic teachings on sex, such as the prohibition of contraception. But I should note that all Christian communions thought contraception was gravely sinful until 1930, when the Anglicans first allowed it in hard cases only. Thus, it is simply historic Christian teaching, not just a “Catholic thing.” It has become the latter because we are the only ones who never forsook the traditional teachings, whereas other Christians decided to reject those.
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The Catholic Church teaches that it is wrong to deliberately separate sexuality from procreation, because the latter is its most fundamental purpose. It’s a natural law argument:
1) The deepest and essential purpose of sex is procreation.
2) Separating sex from procreation is a violation of this purpose and is against natural law.
3) Therefore, whatever does so is sinful and wrong.
God created sex for this purpose and also for pleasure, within its proper sphere (marriage between a man and a woman).
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The Church holds that homosexual orientation itself is not sinful. It is only when these desires are acted upon or excessively dwelt upon (lust), that it becomes sinful. In that respect it’s not that different from heterosexual non-marital sexuality. Men and women after puberty have sexual desires, because God designed it that way, in order for more children to be born. These natural desires need to be controlled and delegated to the proper place and time to find fulfillment. The difference between homosexual and heterosexual sexuality is that the former (when acted upon in the usual ways) is, we believe, contrary to natural law in all circumstances, whereas the latter is sinful outside of marriage and a procreative will, but not sinful within those purviews.
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Bluntly and generally expressed, the Catholic view is that male orgasm must occur within the act of vaginal intercourse with one’s spouse (of the opposite sex) that one is committed to for life, and that female orgasm must also be in conjunction with the overall act of love (intercourse), open to life and possible conception (i.e., no contraceptive devices or intent). Sexual acts that are apart from this circumstance are wrong and sinful. This is Catholic sexuality in its most basic expression, or in a “nutshell.”

The United States Conference of Catholic Bishops approved Create in Me a Clean Heart: A Pastoral Response to Pornography by a vote of 230-4 (and one abstention) at their November 2015 meeting in Baltimore. The document discusses Church teaching on human sexuality and chastity, specifically why the production and use of pornography is always sinful, as well as porn’s disastrous effects on individuals and society as a whole. It also suggests a pathway for those addicted to pornography who wish to break the addiction. The document is primarily addressed to Catholic leaders and parents, but is of value to anyone interested in an authentically Catholic view of the often hidden vice that, directly or indirectly, affects everyone today.
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Regular porn use can destroy a person’s ability to have healthy relationships and successful marriages, the bishops state. Porn is connected to “adultery, domestic violence, the abuse of children in child pornography, and sex trafficking. It also can be implicated in contraception use and abortion, given that it promotes and even celebrates promiscuity and a view of sexuality devoid of love or openness to new life.”

Furthermore, “pornography use within marriage severely damages the spouses’ trust and intimacy both because of the pornography use itself and because of the deception and lies usually involved in one spouse hiding his or her behavior from the other. It has been identified by divorce lawyers as a major factor in over half of divorces.”
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Even though we live in a world torn apart by porn addiction, there is healing or restoration. If you are held captive by this addiction, let go of your shame and reach out for help.